rgw: Keystone token parsing doesn't need to know API version.

rgw: Keystone token parsing doesn't need to know API version.

This patch targets an issue with S3Extension in Keystone:
requested tokens always conform to Keystone API v2 - regardless
of the version used to make the request.

Previous implementation of KeystoneToken::parse() as well
as KeystoneToken::decode_json() had to know the API version
explicitly. Thus, they might be affected in the future by
changes in S3-compatibility middleware of Keystone.

Signed-off-by: Radoslaw Zarzynski <rzarzynski@mirantis.com>